The present invention relates to a gas flow rate measuring apparatus for measuring an intake air flow rate in an engine (internal combustion engine), and specifically, relates to a gas flow rate measuring apparatus suitable for obtaining not only the gas flow rate but also a gas temperature detection signal.
In automotive engines, measurement of the intake air flow rate is necessary to control a fuel injection quantity.
One type of the apparatus for measuring an intake air flow rate is a heat resistor type gas flow rate measuring apparatus. A detection circuit of the heat resistor type gas flow rate measuring apparatus includes a bridge circuit composed of a heat resistor (also called a hot wire), a gas temperature measuring resistor (also called a cold wire), and two fixed resistors. The heat resistor and the gas temperature measuring resistor are placed in an intake gas passage.
The heat resistor type gas flow rate measuring apparatus has a constitution of controlling power supply so as to keep constant temperature of the heat resistor in the bridge circuit to maintain a resistance balance of the bridge circuit.
With such a constitution, heat release rate of the heat resistor is increased in accordance with an increase of the intake gas flow rate in the intake gas passage. Meanwhile, a current is increased so as to keep the temperature of the heat resistor constant. Accordingly, the air flow rate can be measured based on a voltage V2 which appears across the fixed resistor connected in series to the heat resistor.
The voltage V2 is processed in an adjustment arithmetic circuit with a predetermined input-output characteristic to have a required air flow rate-signal characteristic, and then outputted from the adjustment arithmetic circuit as a flow rate signal in a predetermined relationship with the air flow rate.
Incidentally, for an output signal characteristic of the gas flow rate measuring apparatus, it is desired that change in the output signal is small even when the temperature is changed, in other words, a temperature dependent error is small.
The temperature dependent error falls into two main categories: an error dependent on gas temperature and an error dependent on circuit substrate temperature. The gas temperature dependent error is caused by change in the gas temperature while the circuit substrate temperature is constant. The substrate temperature dependent error is caused by change in the circuit substrate temperature while the gas temperature is constant.
In order to reduce the temperature dependent error, it is necessary to compensate the temperature dependent error of the gas flow rate detection signal using detection signals of the gas temperature and the substrate temperature.
With regard to the compensation of the gas temperature dependent error, for example, there is a technology described in a JP-A-11-37815 gazette. In the technology described in the gazette, the intake air passage includes a separate temperature sensor such as a thermistor arranged therein. The gas temperature dependent error is compensated by digital operation using the temperature detection signal thereof.
The thermistor is cheap, but resistance value thereof is an exponential function of an inverse of temperature so that the temperature detection signal is non-linear with respect to the temperature.
Accordingly, operation during the digital operation is complicated, and the circuitry thereof becomes complicated.
Instead of the thermistor, a Pt (platinum) resistor having an output characteristic with a good linearity can be used. However, the Pt resistor is expensive, and the apparatus cost is increased. Accordingly, the Pt resistor is not preferred.
As described above, the gas temperature measuring resistor is arranged in the gas flow rate measuring apparatus. From this perspective, several methods of not only obtaining the air flow rate signal but also outputting the gas temperature detection signal from the gas flow rate measuring apparatus are proposed.
For example, in a technology described in a JP-A-5-164583 gazette, the gas temperature measuring resistor is driven by a constant current to take out a voltage drop, and thus the gas temperature detection signal is obtained. Simultaneously, a multiplier circuit is connected to a constant temperature control circuit of the heat resistor. The input of the constant temperature control circuit is multiplied by the above described voltage drop.
JP-A-7-139985 and JP-A-8-86678 gazettes describe technologies of obtaining the gas temperature detection signal.
Specifically, in the technologies described in the above gazettes, a voltage V1 appearing across a combined resistance of the gas temperature detection resistor and the fixed resistor, or a voltage V1 appearing across a combined resistance of the heat resistor and the fixed resistor, and a voltage V3 appearing across the fixed resistor connected in series between the gas temperature measuring resistor and the ground, or a voltage V2 appearing across the fixed resistor connected in series between the heat resistor and the ground are inputted to a divider circuit composed of an analogue circuit to obtain V1/V2 or V1/V3, respectively, and the gas temperature detection signal is thus obtained.
However, in the above described conventional art, since a multiplier circuit or divider circuit composed of an analogue circuit is used to obtain a gas temperature detection signal, performance of the circuit is widely varied due to production variation, and there has been a fear of increase in man-hour for adjustment for each product.
Moreover, since a temperature dependent error is caused in the analogue circuit, when the temperature at the circuit portion is changed, the output of the gas temperature detection signal could be changed while the gas temperature does not change. Therefore, the man-hour for design or adjustment for reducing the temperature dependent error is increased, and thus manufacturing costs are increased.
Therefore, in order to compensate the output error of the gas flow rate detection circuit with high accuracy, it can be conceived that the gas temperature detection signal is obtained using a digital circuit.
However, if the digital circuit is simply applied for obtaining the gas temperature detection signal, the circuitry becomes complicated and the price thereof is increased.
An object of the present invention is to realize a gas-flow rate measuring apparatus including a digital circuit capable of easily taking out a highly accurate gas temperature detection signal in a simple constitution by a small cost increase.
In order to achieve the above described object, the present invention is constituted as follows.
(1) In an apparatus for measuring a gas flow rate, which includes one or a plurality of resistors arranged in a gas passage and a gas flow rate detection circuit for outputting a gas flow rate detection signal in accordance with a gas flow rate flowing in the gas passage by detecting currents flowing through the resistors or voltages generated in accordance with the currents, the apparatus includes a fixed resistor connected in series to one of the resistors; and a first A/D converter circuit for converting an input voltage into a digital signal and outputting the digital signal by using a voltage generated in the fixed resistor as a reference voltage and using a voltage generated in a combined resistance of the resistor and the fixed resistor as the input voltage, and a digital output signal of a gas temperature signal is obtained by the first A/D converter circuit.
(2) In an apparatus for measuring a gas flow rate, which includes one or a plurality of resistors arranged in a gas passage, and a gas flow rate detection circuit for outputting a gas flow rate detection signal in accordance with a gas flow rate flowing in the gas passage by detecting currents flowing through the resistors or voltages generated in accordance with the currents, the apparatus includes a fixed resistor connected in series to the resistors; a first A/D converter circuit-for converting an input voltage into a digital signal and outputting the digital signal by using a voltage generated in a combined resistance of the resistor and the fixed resistor as the input voltage; a second A/D converter circuit for converting an input voltage into a digital signal and outputting the digital signal by using a voltage generated in the fixed resistor as the input voltage; and a first digital arithmetic circuit for dividing the digital output signal from the first A/D converter circuit by the digital output signal from the second A/D converter circuit, and a digital output signal of a gas temperature signal is obtained by the first digital arithmetic circuit.
(3) In an apparatus for measuring a gas flow rate, which includes a heat resistor arranged in a gas passage; a first fixed resistor connected in series to the heat resistor; a gas temperature measuring resistor arranged in the gas passage; a second fixed resistor connected in series to the gas temperature measuring resistor; current control means for controlling a current flowing through a bridge circuit including the heat resistor, the first fixed resistor, the gas temperature measuring resistor, and the second fixed resistor; and a gas flow rate detection circuit for outputting a gas flow rate detection signal in accordance with a gas flow rate flowing in the gas passage based on the current flowing through the bridge circuit, the apparatus includes an A/D converter circuit for converting an input voltage into a digital signal based on a reference voltage and outputting the digital signal by using any one of a voltage at a junction of the heat resistor and the first fixed resistor and a voltage at a junction of the gas temperature measuring resistor and the second fixed resistor as the reference voltage, and using a voltage generated in a combined resistance of the gas temperature measuring resistor and the second fixed resistor as the input voltage, and a digital output signal of a gas temperature signal is obtained by the A/D converter circuit.
According to the invention described in the above (1), the first A/D converter circuit is effectively used, and has a function substantially similar to a divider, so that the divider becomes unnecessary. Accordingly, the digital value of the gas temperature signal can be calculated independently of changes in the reference voltage and the input-voltage in a simple constitution.
Moreover, since the digital value of the gas temperature signal is configured to be linear with respect to change in the gas temperature, a table or the like becomes unnecessary.
Accordingly, the gas flow rate measuring apparatus can be realized, which includes a digital circuit capable of taking out a highly accurate gas temperature detection signal easily in a simple configuration with minimizing cost rise.
According to the invention above described (2), various circuitries for obtaining a digital value of the gas temperature signal can be conceived. With the constitution as the above (2), the circuitry can be simple, and the digital value of the gas temperature signal can be calculated independently of changes in the reference voltage and the input voltage.
Accordingly, the gas flow rate measuring apparatus can be realized, which includes a digital circuit capable of taking out a highly accurate gas temperature detection signal easily in a simple configuration with minimizing cost rise.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.